At the present time, there are a number of railroad equipment manufacturers which provide the railroad industry with "metal to metal" type rubbing constant contact side bearing assemblies. These constant contact side bearing assemblies must meet the American Association of Railroads (AAR) specification (M-948-79) dated Apr. 4, 1984, and published in the Manual of Standards and Recommended practices. The teachings in this specification are incorporated herein by reference thereto. These constant contact side bearing assemblies are also recognized in the railroad art as resilient side bearings. The term resilient side bearing assembly will be used hereinafter in the description of the invention.
According to the above-referenced specification, the function of the resilient side bearing assembly is to act as a resilient or constant supplemental support between the car body and truck, and offer the means for transmitting car body rocking forces into the truck system throughout a truck swivel range of eleven degrees in either direction.
In order to accomplish this function, the resilient side bearing must operate within the five and one-sixteenth inch nominal working height between the truck bolster and the body side bearing wear plate secured to the underside of the car body. This side bearing wear plate is located two feet-one inch from the center line of the car. In addition, the resilient side bearing must having the capacity to sustain, without permanent deformation, impact (rocking) forces equal to the vertical side bearing load P times a constant. This constant is 1.43. Furthermore, the resilient side bearing assembly must be secured to the truck bolster and be able to withstand a design shear force equal to P.times.1.43.times.the coefficient of friction. In recovery from deformation, vertical force and contact must not be lost.
It is also a requirement that the resilient side bearing assembly preload must be equally distributed and its torsional resistance, when combined with the resistance of the center plate and any other devices with which the car may be equipped, must not interfere with the ability of the car to negotiate the minimum radius curve for which it is designed.
In this specification, the term "pretravel" of a resilient side bearing assembly is the travel from the free height to the five and one-sixteenth inc installed height. As the car sways from side to side, the bearing on the car's high side could have as much as an eleven-sixteenth inch increase in height. Although it is desirable for the friction head portion of the resilient side bearing assembly to stay in contact with the car body wear plate, it is difficult t achieve this amount of pretravel with the resilient side bearings of the prior art of which applicant is aware.
One such resilient side bearing is marketed by Miner Enterprises, Inc. under the tradename Tecs Pac. This resilient side bearing assembly consists of three parts: a metal housing and a metal cap attached to an elastomer pad. The A. Stucki Company also provides a resilient side bearing assembly. This resilient side bearing assembly also consists of: a metal housing and a metal cap with resilient elements having sloped upper surfaces into which are keyed mating sloped surfaces of a corresponding metal cap. This inclined interface between the resilient elements and the metal cap provides a wedging action which eliminates the longitudinal force motion in the assembly. A steel roll is disposed between the resilient elements, and a pair of hardened steel end closures are used to close the cage and openings and to contain the resilient elements.
The resilient element in the Miner resilient side bearing assembly is a Hytrel elastomer, manufactured by the DuPont Company, while the Stucki resilient side bearing assembly utilizes urethane for these resilient elements.